The present invention relates to methods and apparatus for electromagnetic well logging, and more particularly to well logging employing a novel, deconvolution technique, adaptable to accurate measurement of formation conductivites in a wide variety of formation environments.
In induction well logging, a coil system is lowered into a well bore for the purpose of investigating the electrical properties of earth formations adjacent the well bore. An electrical property of interest in such investigation is the electrical conductivity of particular portions of the formation.
Typically, induction logging coil systems include at least one transmitter coil and one receiver coil, though plural coils or coil arrays are often employed in place of a single transmitter coil or single receiver coil. A time varying signal is impressed on the transmitter coil or coil array and a signal is received by the receiver coil or coil array. The received signals are a function of the coil system structure and the coil system environment, which, of course, includes formation portions of interest.
It is an object of the present invention to approximately determine the true conductivity of incremental horizontal layers of the formation immediately adjacent to the induction tool coil system and compensate for undesired effects on the measured signal due to the presence of nearby layers or strata of arbitrary thicknesses and conductivities.
It is known in the prior art to measure and record data obtained by an induction coil system at a plurality of depths and combine predetermined fractions of the signals to produce an adjusted value for the conductivity of the formation. Such a technique is taught in U.S. Pat. No. 3,166,709 to Doll. To do this Doll defines so-called "geometric factors" which are said to depend exclusively on the dimension and position of the coils with respect to the formation. However, the apparent conductivities measured by the induction coil array differ significantly from those estimated by using the Doll geometric factors particularly in high conductivity formations (i.e., greater than 1 mho per meter) as indicated in Thadani and Hall, Jr., "Propagated Geometric Factors In Induction Logging," presented at the 22nd Annual SPWLA Symposium, June 1981.
A variable frequency well logging system (FIGS. 1 and 1a) which obtains accurate conductivity measurements through wide variations in formation conductivity, without requiring instrumentation with wide dynamic range, is the subject of the patent application of Bravenec, Hall, Merchant and Thadani filed on an even day herewith and assigned Ser. No. 457,149. A second variable frequency well logging system (FIG. 2), designed to minimize propagation effects, is part of the subject matter of this patent application.
The aforementioned Doll technique does not provide a rigorous means for determining true conductivity and delineating accurately layers of selected thickness for the useful, general case of a cylindrically symmetric formation having an arbitrary arrangement of layers or strata of different, arbitrary thicknesses and conductivities because propagation effects are ignored.
It is an object of the present invention to provide such a technique.
These and other objects and features of the present invention will become apparent from the claims and from the following description when read in conjunction with the accompanying drawings.